This invention is directed to an optical waveguide which allows the phase front of light propagating in an optical waveguide to be controlled by an electrical signal.
The propagation of light in optical waveguides is of current interest because these systems can perform many dataprocessing and communication functions. Integrated optical technology may be envisioned to combine a system that is capable of modulating, switching, and detecting with optical microcircuitry. A microoptical system is desirable since it is rigid, free from environmental effects; and capable of handling greater volumes of information than traditional electronic systems.
Optical waveguides are well known in the art. An optical waveguide consists of a region in which the index of refraction is greater than the index of refraction of the surrounding medium, top and bottom. Such has been set forth in an article "Light Waves In Thin Films and Integrated Optics", P. K. Tien, Applied Optics, 10, page 2395, 1971. Light propagating along the waveguide region will suffer total internal reflection at the boundries between the waveguide region and the surrounding medium if the angle of incidence .theta. is greater than the critical angle .theta..sub.c, that is, .theta.&gt;.theta..sub.c = Sin .sup.-.sup.1 .sub.(N.sbsb.0,.sbsb.2/N.sbsb.1) where the index of refraction of the upper layer is N.sub.0, the waveguide region N.sub.1 and the lower layer N.sub.2. The light traveling in the waveguide is trapped in the waveguide region by being totally reflected from the upper and lower interfaces between the waveguide region and the upper and lower mediums. There is considerable discussion in the literature concerning various topological shapes of optical waveguides and their characteristics. Waveguides may be flat slabs, rectangular, cylindrical, or any other shape. Metal-clad dielectrics may not be used in optical waveguides due to the high losses. When light is reflected off a metal surface, the energy losses depend on the metal, the wavelength of the light, surface condition, polarization, and angle of incidence, and losses are typically about 1 or 2 percent per reflection. Hence, metal-clad waveguides, because of the number of reflections per centimeter, are extremely lossy in the optical region. In the dielectricclad waveguide, losses are due to absorption in the dielectric and scattering losses. If the cladding has no absorption at the optical wavelength being propagated then no energy is absorbed from the evanescent wave (extending into the cladding) and the waveguide suffers only from scattering losses.
In directing light through an optical propagation path such as the atmosphere, the wavefronts can be distorted by irregularties in the optical path. Heretofore, in attempting to correct for these distortions, the laser beam has been divided into, N components and each component is passed through an electrooptic phase modulator. These modulators are bulk effect devices which are generally expensive, they require complex electronics to drive them to produce the desired phase function across the optical beam. Further, to obtain a fine degree of control, N, must be large and the phase sources must be spatially placed close together. For phased array beam steering over large angles, the spacing would have taken on the order of .lambda., the wavelength of the radiation.